Detecting Test Gas Fluctuations During Sniffer Leak Searching

ABSTRACT

A method for detecting fluctuations in the amount of test gas detected by a sniffer probe of a leak detector in the gas flow from around a test piece pressurized with an oxygen-free test gas containing at least an amount of CO 2 , wherein an amount of oxygen in the ambient air is measured.

The invention relates to a method and to a device for detecting and compensating fluctuations in the gas flow taken in with a sniffer probe of a leak detector from the atmosphere around a test piece pressurized with CO₂ as test gas.

Sniffer leak detection is an established method, especially for localizing leaks. For sniffer leak detection, a test gas is used to apply a positive pressure to a hollow body (test piece) to be tested for tightness. Air from the locations of the test object to be tested is taken in by a sniffer probe. If leak gas from the test piece escapes at the tested location, it is taken in with the air flow and causes an increase in the test gas concentration in the intake air flow. This increase in concentration is evaluated as a measure for the leak rate. In addition, the partial pressure of the test gas is measured with a suitable test gas detector. This detector is disposed at a suitable location in the testing system.

The detector may be positioned immediately at the tip of the sniffer probe, or it may be arranged in the grip or even in the main device of the leak detector upstream or downstream of the gas-supplying unit (pump, compressor). The gas-supplying unit produces the gas flow taken in by the sniffer probe.

If the concentration of the test gas in the testing area is constant, the following is true for the total concentration in the sniffer gas flow at the leak location:

$c = {{\frac{Q_{Leckage}}{Q_{FL}} \cdot \left( {1 - c_{0}} \right)} + c_{0}}$

Q_(Leakage) Test gas leak rate

Q_(FL) Sniffer gas flow

c₀ Constant offset of test gas in the air

c Effective CO₂ concentration in the sniffer gas flow

c₀ is therefore the starting concentration of the test gas in the intake gas flow (carrier gas flow). c is the test gas concentration that contains the amount of test gas escaping from the leak. Q_(Leakage) is the escape rate of the test gas at the leak. Q_(FL) may also be called the carrier gas flow.

Sniffer leak detection when using the test gas CO₂ is strongly negatively influenced by the CO₂ fluctuations in the ambient concentration. The CO₂ concentration of “fresh air” is about 400 ppm. However, this concentration is increased by various CO₂ emitters, for instance breathing gas of users, exhaust gas from internal combustion engines, etc.

Oxygen is used when fats, proteins, and carbohydrates are burned in the body of the user; for example, the chemical reaction equation for oxidation of glucose (sugar) is: C₆H₁₂O₆+6 O₂→6 CO₂+6 H₂O+energy.

This instability in the ambient concentration profoundly limits the smallest detectable leak rate.

The underlying object of the invention is to detect fluctuations in test gas in the gas flow taken in with a sniffer probe of a leak detector.

The method according to the invention is defined by the features of patent claim 1. The device according to the invention is defined by the features of patent claim 6.

A gas that is as free of oxygen as possible and that has an amount of CO₂ is used for the test gas. In addition to CO₂, the test gas may also have other amounts that are oxygen-free. The test gas may be CO₂ in particular. What is critical is that an oxygen amount in the test piece filled with test gas is negligible or does not exist.

The invention is thus based on the underlying thought of detecting the amount of oxygen in the atmosphere around the test piece and where possible in the intake gas flow of the sniffer probe. This oxygen amount is to act as evidence of that amount of CO₂ in the intake gas flow that does not result from a leak in the test piece. In addition, it may be assumed by approximation that a decrease in the oxygen concentration in the ambient atmosphere is proportional to a corresponding increase in the CO₂ concentration. The measurement may be performed with a mass spectrometer or another sensor that measures the test gas partial pressure. The measurement of the amount of oxygen is preferably performed using a lambda probe and if possible at atmospheric pressure.

The oxygen concentration in the breath gas of the person operating the leak detector is less than that of the ambient atmosphere and the CO₂ amount is higher. If the breath gas of the person operating the equipment is taken in by the sniffer probe, the amount of CO₂ in the intake gas flow increases, which would lead to inaccurate measurement results if the test piece is pressurized with CO₂ as the test gas. The CO₂ probe of the leak detector cannot judge whether an amount of CO₂ results from a leak in the test piece or from the ambient atmosphere, for instance from the breath gas of the operator. This amount of CO₂ in the ambient atmosphere that corrupts the measurement and is not the result of a leak in the test piece is called the offset in the following. This offset may also result, for example, from the exhaust gas of internal combustion engines.

The amount of O₂ in the ambient atmosphere or in the gas flow taken in by the sniffer probe is determined using the oxygen probe. This amount of oxygen cannot come from a leak in the test piece if the test piece is filled with oxygen-free test gas, for instance, if it is filled exclusively with CO₂ as the test gas. Thus it is possible to determine the offset amount of CO₂ from the measured amount of oxygen.

This offset c₂ (t) constitutes a constant offset c₀ in the intake air and a time-dependent fluctuating offset c₁ (t) of test gas in the intake air. The offset c₂ (t) may be calculated as follows:

Q_(Leakage) Test gas leak rate

Q_(FL) Sniffer gas flow

Q_(Leakage)<Q_(FL)

c₀; c₁; c₂<1

c Effective CO₂ concentration in the sniffer gas flow

c₀ Constant offset of test gas in the air

c₁ (t) Time-dependent offset of test gas in the air

c₂ (t)=c_(o)+c₁ (t) Total offset of test gas in the air

a Ratio factor between O₂ concentration and CO₂ concentration

b Sensitivity factor O₂ of the lambda probe

Δ I Signal of the lambda probe

Δ c Change in the O₂ concentration

${\Delta \; {c_{1}(t)}} = {a \cdot \frac{1}{\Delta \; c_{o_{2}}}}$ Δ c_(o₂) = b ⋅ Δ I $Q_{Leckage} = {Q_{FL} \cdot \left\lbrack \frac{c - {c_{2}(t)}}{1 - {c_{2}(t)}} \right\rbrack}$ ${{where}\mspace{14mu} {c_{2}(t)}} = {c_{0} + {\frac{a}{b} \cdot \frac{1}{\Delta \; {I(t)}}}}$

An exemplary embodiment of the invention shall be explained in greater detail in the following. The FIGURE is a schematic representation of an exemplary embodiment of the device according to the invention.

The test piece 12 may be, for example, food packaging. The test piece 12 is pressurized with positive pressure with respect to the ambient atmosphere 14 and filled oxygen-free with at least CO₂ test gas. The test gas flows out through a leak 16 due to the positive pressure relative to the ambient atmosphere 14.

The leak detector 18 works according to the principle of sniffer leak detection and to this end has a sniffer probe 20 that is connected to a pump 22 or a compressor 22.

The pump 22 or compressor 22 produces a gas flow that is taken in through the inlet 24 of the sniffer probe 20. The leak detector 18 furthermore has a partial pressure sensor 26, in the form of a mass spectrometer, that reacts to CO₂ and that determines the amount of CO₂ test gas in the intake gas flow of the sniffer probe 20. If the sniffer probe 20 is moved along the outer surface of the test piece 12 towards the leak 16, the amount of test gas in the intake gas flow increases, which may be detected using the partial pressure sensor 26.

The amount of CO₂ in the intake gas flow may rise for other reasons, however, for instance if the breath gas of the person moving the sniffer probe 20 is taken in by the sniffer probe or if the sniffer probe 20 is disposed in the vicinity of an exhaust gas flow of an internal combustion engine. In this case, the partial pressure sensor 26 detects a rising amount of CO₂. In order to prevent this from incorrectly being understood as suggesting a leak 16 in the test piece 12, according to the invention an oxygen sensor 28 that measures the amount of O₂ in the intake air flow is provided.

The oxygen sensor 28 may be a sensor that detects the partial pressure of oxygen, a mass spectrometer, for example, or it may even be, for example, a conventional lambda probe.

The oxygen sensor 28 in the depicted exemplary embodiment is arranged in the gas flow between the sniffer probe 20 and the pump 22. Alternatively, the oxygen sensor may be provided directly in the grip 20 or may measure the exhaust gas flow of the pump 22 at its outlet.

The amount of oxygen measured is used to detect that amount of CO₂ in the intake gas flow that does not derive from a leak 16 in the test piece 12, but rather results from combustion of oxygen. 

1. A method comprising detecting fluctuations in an amount of test gas detected by a sniffer probe of a leak detector in a gas flow from ambient air surrounding a test piece pressurized with an oxygen-free test gas containing at least one amount of CO₂, wherein an amount of oxygen in the ambient air is measured.
 2. The method according to claim 1, wherein the amount of oxygen measured is used to determine an amount of CO₂ in the ambient air that does not result from a leak in the test piece.
 3. The method according to claim 1, wherein the measurement of the amount of oxygen in the ambient air surrounding the test piece is taken at atmospheric pressure.
 4. The method according to claim 1, wherein the amount of oxygen in the gas flow of the sniffer probe or an exhaust gas flow of a gas-conveying pump of the leak detector connected to the sniffer probe is measured.
 5. The method according to claim 1, wherein an offset c₂ (t) of the amount of test gas that is contained in an intake gas flow and that does not derive from a leak in the test piece is determined using the equation: ${{c_{2}(t)} = {c_{0} + {c_{1}(t)}}},{{{where}\mspace{14mu} {c_{1}(t)}} = {\frac{a}{b} \cdot \frac{1}{\Delta \; {I(t)}}}},$ wherein c₀ is a constant offset of CO₂ in the intake gas flow, c₁ (t) is a time-dependent offset amount of CO₂ in the intake gas flow, a is a ratio factor between measured oxygen concentration and existing CO₂ offset concentration, b is a sensitivity factor for the amount of oxygen measured by an oxygen probe, and ΔI(t) is a change in a measurement signal of the oxygen probe.
 6. A device for sniffer leak detection, comprising: a sniffer probe of a leak detector, wherein the sniffer probe comprises a gas-conveying pump for taking in and measuring a gas flow that flows out of a test piece and into an ambient atmosphere surrounding the test piece, the test piece pressurized with a test gas relative to the ambient atmosphere, and wherein the test gas is oxygen-free and has at least an amount of CO₂; and an oxygen sensor for measuring an amount of oxygen in the gas flow taken in by the sniffer probe.
 7. The device according to claim 6, wherein the oxygen sensor is a lambda probe.
 8. The device according to claim 6, wherein the oxygen sensor is a sensor that detects a partial pressure of oxygen.
 9. The device according to claim 6, wherein the oxygen sensor is arranged and embodied upstream of the pump, to measure gas flowing through the sniffer probe, or is arranged and embodied downstream of the pump, to measure an exhaust gas flow of the gas-conveying pump.
 10. The device according to claim 8, wherein the oxygen sensor is a mass spectrometer. 